Anticipatory fear in response to threatening cues readily prompts defensive activation in preparation for responding to an impending aversive event. A hallmark of pathological anxiety, especially fearful disorders, is excessive and persistent emotional responding in anticipation of feared stimuli. The intense fear and anxiety associated with the anticipation of a phobic event often leads to avoidance behaviors that can interfere with daily life. This is especially the case regarding phobias involving fear of pain, where fearfulness often negatively impacts health through delays in preventive care. While considerable advances have been made in understanding the neurocircuitry involved in the acquisition and expression of fear, an important translational step is in determining the neural underpinnings of fear inhibition and perseverative responding. Recent studies using verbal instructions to associate a neutral stimulus with the potential of electric shock have found that this simple threat prompts physiological reactions consistent with defensive activation. Dubbed instructed fear, the plasticity of this form of fear learning has not yet been explored. The proposed research involves comparisons of functional brain activity and psychophysiological responses to visual cues that signal the possibility of electric shock ("threat") or not ("safety"). In an initial learning phase fear will be verbally instantiated through instructions that associate one property of a set of visual cues (e.g. color) with threat or safety. Over the course of the experiment a second set of instructions will shift the instructed fear contingency to a different perceptual dimension causing half of the cues to reverse their affective meaning. Measures of autonomic and somatic reactivity and functional magnetic resonance imaging (fMRI) will be used to assess physiological and neural indicators of fear plasticity, and to gauge the involvement of orbitofrontal cortex in signaling reversals of conditioned fear. Additional hypotheses address whether increases in dental fear impair or enhance fearful responses during reversal learning.